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A new study from researchers in Russia and the UK investigated the use of novel biochemical modifications to antisense oligonucleotides (ASOs) for the treatment of spinal muscular atrophy (SMA). The article, published in Nucleic Acid Therapeutics, describes the production of ASOs with novel backbone modifications and evaluated their splice-switching activity in SMA fibroblasts and murine models of SMA.

Study results in SMA-patient derived fibroblasts showed that 2′-O-(2-methoxyethyl) (2′-MOE) oligonucleotides with internucleotide methanesulfonyl (mesyl, μ) phosphoramidate groups (fully abbreviated as 2′-MOE mesyl) were equivalent to nusinersen, which has a phosphorothioate (PS) backbone. Previous research from the team showed that mesyl phosphoramidate oligonucleotides were “demonstrated to possess increased specificity, reduced toxicity, and improved activity over their PS analogs,” according to the study authors.

Despite the positive results in fibroblasts, further investigation of the 2′-MOE mesyl ASO equivalent of nusinersen in a neonatal mouse model of SMA showed it to be inferior to the corresponding PS ASO in terms of survival (median survival 18 days versus 222, respectively) when given at the same concentrations of 40 mg/kg. There were no statistically significant differences in pup weight between the two treatments, however.

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Hammond et al. postulated that the lower activity of the 2′-MOE mesyl ASO may have been due to reduced release from endosomes and/or uptake by the nucleus. This hypothesis was supported by fluorescent confocal microscopy in the HEK293 cell line which showed that 2′-MOE mesyl ASO endosomal release levels were roughly 2 times less than the PS ASO and nuclear localization was decreased by ~3 to 4 times in comparison.

These results suggest that there may be different intracellular trafficking and nuclear import pathways between the two ASOs. Further investigation into these pathways may aid in the design of ASOs with different mode-of-action targets such as cytoplasmic versus nuclear.

Timofei Zatsepin, one of the study’s co-authors and an associate professor at the Skolkovo Institute of Science and Technology (Skoltech) in Russia, stated “As µ-oligos [mesyl oligonucleotides] are more stable and less toxic in vivo than PS oligos, we propose that µ-oligos used in higher doses can provide the same efficacy together with more prolonged action – this study is under development now.”


Hammond SM, Sergeeva OV, Melnikov PA, et al. Mesyl phosphoramidate oligonucleotides as potential splice-switching agents: impact of backbone structure on activity and intracellular localization. Nucleic Acid Ther. 2021;31(3):190-200. doi:10.1089/nat.2020.0860
New components for antisense gene therapy show promise in treating spinal muscular atrophy. Skoltech. Published June 29, 2021. Accessed July 12, 2021.